共价有机聚合物衍生的Si@NC应用与高性能锂离子电池负极

[27-REF_Ref163936005\r\h\#"0]"28]。在扫描第二圈、第三圈时氧化还原尖峰的消失，表明SEI膜趋于稳定，在第二次和第三次的循环过程中，在0.38V、0.54V的氧化峰与锂从高浓度转化为低浓度直至非锂的转化过程有关，这一步可能涉及到电极反应为：LixSi→Si+xLi++xe−，且后两条曲线的重合度较高，说明了Si@NC-2的可逆性以及稳定性较好。图5Si@NC-1、Si@NC-2、Si@NC-3复合材料的交流阻抗谱图图5中存在的曲线分别为SiNPs和Si@NC-1、Si@NC-2、Si@NC-3复合材料做锂离子电池负极时的交流阻抗谱（EIS）图。由图可以明显看到交流阻抗曲线是由一个半圆和一条直线组成的一条平滑曲线，且曲线前段的半圆和后段的直线相切，该切点的横坐标即为所求的电池阻抗。从图中得Si@NC-1、Si@NC-2、Si@NC-3的阻抗分别为200Ω、250Ω和300Ω，由此可得Si@NC-3的阻抗最大、Si@NC-1的阻抗最小，但都远小于SiNPs的阻抗值。故而可以推导出Si@NC材料的阻抗值随所加入的SiNPs的量而增大，但由COPs包覆所形成的核壳结构、COPs煅烧后的核壳中存在的N元素以及大量的碳元素的掺入极大地降低了Si的阻抗，增强了目标材料的导电性。阻抗值越低、锂离子的传导越快，电池的可逆性以及稳定性越高。图6Si@NC-1、Si@NC-2、Si@NC-3复合材料在电流密度为0.1Ag-1的循环性能图。为了研究锂离子电池的锂离子储存性能，在0.1Ag-1的电流下Si@NC-1、Si@NC-2、Si@NC-3电极在循环100圈后的循环性能如图6所示。SiNPs电极循环7次后放电容量迅速衰减至110mAhg-1，这可能是因为SiNPs电极发生破裂和粉碎。得益于COPs封装形成的核壳结构使Si@NC纳米复合材料的放电电容得到了极大的改善，在0.1Ag-1电流的循环下，Si@NC-2电极的初始放电容量为2154.9mAhg-1，在循环100圈后，该电极的放电容量基本稳定在1164mAhg-1，表现出优异的循环稳定性。然而，Si@NC-1与Si@NC-3电极在循环100圈后的放电容量分别为706.5mAhg-1和822mAhg-1，循环性能远低于Si@NC-2电极。这可能是因为Si@NC-1电极包覆SiNPs的量过少，无法提供足够的容量。但是当包覆过多的SiNPs容易造成剧烈的体积膨胀，电池性能迅速降低。实验结果表明，包覆适量的SiNPs是提高Si@NC复合材料储锂性能的关键。

图7Si@NC-1、Si@NC-2、Si@NC-3复合材料倍率性能图（a），及Si@NC-2复合材料恒流充放电曲线（b）。图7a测试的是Si@NC电极的速率性能，在0.1Ag-1、0.2Ag-1、0.5Ag-1、1.0Ag-1、2.0Ag-1的电流下分别循环10圈后，可以发现电池的放电容量越来越低，Si@NC-2在不同电流密度下放电比容量分别为1179.9mAhg-1、831.7mAhg-1、654.8mAhg-1、560.6mAhg-1和371.7mAhg-1。但在电流密度恢复到0.1Ag-1时，循环10圈后该电极的放电容量依然维持在804.3mAhg-1，表现出优异的倍率性能。虽然经过循环60圈后Si@NC-1和Si@NC-3电极的放电容量分别为680.8mAhg-1和616.1mAhg-1相较初始放电比容量有所下降，但与目前商用的石墨材料做电池负极的材料相比容量较高。图7b为Si@NC-2材料在0.1Ag-1电流密度下的恒流充放电曲线。在第一次放电中存在两个转折点，0.2V处的转折点是由于LixSiy合金的生成，0.8V处的转折点是SEI膜的生成导致的，第二、三圈的放电过程的转折点表明了Si和Li+可逆的合金化过程。第二、三圈的充放电曲线重合度较高，表明了电极的良好可逆性以及稳定性。在循环100圈后，电池的理论容量稳定在1164mAhg-1左右，与图6所表现的结果相符合。结合上述所有数据以及分析可以得到结论：目标材料成功地缓解了Si在循环过程中的体积膨胀，且极大地改善了Si的导电性，提供了更多的活性位点以便于Li+相结合。由Si@NC-2纳米复合材料作为负极材料而制得的锂离子电池具有更大的比容量以及较高的可逆性和循环稳定性。

结论总之，本论文报道并合成了一种N掺杂多孔碳包覆SiNPs的核壳结构纳米复合材料（Si@NC）。该材料以共价有机聚合物作为前驱体，通过在SiNPs表面进行包覆得到。Si@NC纳米复合材料不仅为Li+的运输提供了更多结合位点，而且其放电容量高、合成方法简单，有效地提高了锂离子电池的循环性能和倍率性能。本文所述的硅基材料和碳材料复合方法为制备新型锂离子电池负极材料提供了新思路。

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